Week Number Calculator
Enter a date and instantly see its ISO 8601 week number, what day of the year it is, which quarter it falls in, and the day of the week. The ISO week number follows the international standard where weeks start on Monday and Week 1 is the week containing the first Thursday of the year.
ISO week numbers are commonly used in business planning, manufacturing schedules, and European date systems. They differ from the US convention where weeks start on Sunday, which is why a dedicated tool is helpful — the rule for which week is Week 1 is not intuitive.
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By The Paper Room Editorial Team — Date Tools
Frequently asked questions
What is an ISO week number?▼
ISO 8601 defines weeks as starting on Monday. Week 1 is the week containing the year's first Thursday — equivalently, the week containing January 4. This means some days in early January may belong to the last week of the previous year, and some days in late December may belong to Week 1 of the next year.
Why does January 1 sometimes show as Week 52 or 53?▼
In the ISO system, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year. This is correct per the standard — Week 1 must contain the first Thursday of the year.
How is 'day of year' calculated?▼
It is the number of days elapsed since January 1 of that year, inclusive. January 1 is day 1, February 1 is day 32, and December 31 is day 365 (or 366 in a leap year).